1. Field of the Invention
The present invention relates to a current generator circuitry and more particularly, to a reference current generator capable of providing a reference current with substantially small temperature dependence by using two kinds of resistance, which have different temperature coefficients.
2. Description of the Prior Art
In an integrated circuit a number of amplifier stages are coupled to a constant dc current generated at one location and reproduced at many other locations for biasing the different transistors in the circuit. A popular circuit building block for accomplishing current reproduction is the current mirror showing in FIG. 1. It consists of four matched transistors M1, and M2, M3, and M4 as well as two diodes D1, D2 and one resistor R1. The PMOS transistor M2, NMOS transistor M4 and diode D1 are in series connected and coupled between a voltage supply and a first reference voltage. On the other hand, the PMOS transistor M1, NMOS transistor M3 resistor R1 and diode D1 are in series connected in a similar way and coupled between the voltage supply and a second reference voltage. The second reference voltage can optionally the same as the first reference voltage. The gates of the PMOS transistors M2 and M1 are connected each other and also connected to a drain of the PMOS transistor M1. Moreover, the gates of the NMOS transistors M3 and M4 are connected together and also to a drain of the NMOS transistor M4 so that it ensures NMOS transistor M4 in the active mode.
The reference current Iref generated can be expressed as
Iref=(kT/q*In(A2/A1))/R1,xe2x80x83xe2x80x83(1)
Where k is the Boltzmann""s constant, T is absolute temperature, and q is the electric charge, and A1 and A2 are the diode areas of D1 and D2, respectively. In the equation (1), the resistance R1 is inherently temperature dependent and has temperature coefficient Tc. Thus the current Iref has a temperature dependent not only on the term kT/q but also on the denominator, the resistance R1. While designing a current generator, it is of great vital that the current generator Iref is independent from the power supply as well as the temperature variations.
An object of the invention is thus to solve aforementioned issues.
The current generator circuitry for providing a reference current with small temperature dependence feature is disclosed. The circuitry comprises a first and a second PMOS transistor, a first and a second NMOS transistor, a first and a second diode, as well as a first and a second resistors. The first PMOS transistor, the first NMOS transistor and the first diode are in series connected between a power reference and a potential reference. It flows with a primary current. The second PMOS transistor has a gate terminal connected to a gate of the first PMOS transistor thereto connect to a drain terminal of the second PMOS transistor. Furthermore, the second NMOS transistor has a gate terminal connected to a gate of the first NMOS transistor thereof connecting to a drain terminal of the first NMOS transistor. The second PMOS transistor, the second NMOS transistor, the second diode, the first resistor and the second resistor are in series connected between the power reference and the potential reference to flow a reference current. Worth to note, the first resistor has a small temperature coefficient and the second resistor has a large temperature coefficient so that the average temperature coefficient is close to a critical value, 3.33E-3. As a result the reference current generator has a feature of very small temperature dependence.